A solenoid actuator for driving hydraulic equipment such as a valve is constituted, for example, by the following members.
Specifically, two cylindrical magnetic path forming members made respectively in a cylindrical form are arranged coaxially with a gap provided there-between in an axial direction, and a plunger made of a magnetic material is disposed on the inside of the magnetic path forming members. A coil provided on the outside of the magnetic path forming members is energized to form a magnetic path through the magnetic path forming members. According to this arrangement, magnetic flux transfer between the two magnetic path forming members is performed via the plunger such that the plunger is driven in the axial direction by a magnetic force, and a shaft fixed to the plunger performs a linear motion.
Working oil in the hydraulic equipment is introduced into the solenoid actuator to lubricate bearings supporting the shaft which performs a linear motion and obtain a preferable balance of pressures acting on the bearings in the axial direction.
However, when working oil containing a contaminant such as abrasion powder generated in the hydraulic equipment is introduced into the solenoid actuator, the contaminant tends to be deposited in a strong magnetic field portion of the actuator. The contaminant deposited in the strong magnetic field portion adversely affects the operation of the solenoid actuator by weakening a thrust which the solenoid exerts on the shaft or increasing a sliding resistance of the shaft with respect to the bearings.
To prevent the contaminant from depositing in the strong magnetic field portion, JP2006-064076A, issued by the Japan Patent Office in 2006, proposes an annular member which allows the shaft to be free to slide while blocking working oil flowing from the hydraulic equipment from invading a plunger chamber formed between the bearings.
For the same purpose, JPH11-031617A, issued by the Japan Patent Office in 1999, proposes a ring filter facing the plunger chamber so as to be penetrated by the shaft, and a contaminant pool formed on the opposite side of the ring filter to the plunger chamber.